Species-specific hybridization probes will be constructed that are capable of distinguishing the LINES ONE sequences of Mus domesticus and Mus spretus. This is possible because concerted evolution of the LINE ONES repeats causes them to accumulate species-specific mutations. Oligonucleotide probes that cover these species- specific mutations will be synthesized. Several such probes have been synthesized and characterized to document the feasibility of this approach. Within 60 kilobases of any gene in the mouse genome there should be a species-specific marker that is detectable by these probes. This marker system will be used with Mus domesticus/Mus spretus hybrids as a general tool for mapping and isolating genes. It is proposed to use the species-specific probes to address the W and Steel loci. Mutations at these two loci both cause developmental defects in blood cells germ cells, and pigment cells. W mutants are thought to carry defects in the respective stem cells; Steel mutants are thought to be defective in the environment with which the stem cells must interact. The experimental strategy requires the breeding of congenic strains carrying Mus spretus DNA at the target locus on a Mus domesticus background or vice versa. Arrangements have been made to gain access to such hybrids the species-specific probes will be used to isolate clones from the genomic sequences in the congenic region. The use of species-specific probes for following the segregation of the two parental genomes in Mus spretus/Mus domesticus hybrids is also described. It is proposed to monitor the reduction in the parental loss of particular markers, or the chromosomal configuration could be monitored. Choosing the offspring to parent each successive generation based on this information would increase the speed and range of genetic techniques involving Mus spretus/Mus domesticus hybrids.